[3] Early in his career, Felder served as chief of staff to Democratic New York State Assembly member Dov Hikind.

[7] During his tenure on the City Council, Felder gained popularity among his conservative Jewish constituents for advocating strongly on their behalf and for supporting Israel.

[8] Although personally opposed to homosexuality as being against Orthodox Judaism, he backed Christine Quinn for City Council Speaker in what was seen as a political move to gain allies.

[11][12] Felder has also gained attention by calling for better labeling of caffeine content in foods and beverages,[13] as well as a ban on the distribution of unsolicited fliers.

[15] In 2005, Felder crossed party lines to endorse then-Republican Mike Bloomberg in his bid for reelection as New York City mayor.

As a member of the Council's Land Use Committee, Felder was part of a group of New York state legislators that has consistently blocked plans to renovate United Nations headquarters, calling the UN anti-American and anti-Israel.

Even though incumbent David Storobin had won the seat in a special election, the GOP offered little support to him, and Felder handily defeated him.

[3] Brooklyn Democratic Party Chairman Frank Seddio called Felder's defection a "disgrace and a complete betrayal of his constituents.

He cast deciding votes against an immigration bill called the New York Dream Act, stated that ending stop-and-frisk was a mistake, opposed plastic bag fees, and supported Iran divestment and anti-BDS laws.

[35] In April 2018, other lawmakers accused Felder of "essentially holding the $168 billion budget hostage until the state agreed not to interfere with the curriculum at the private Jewish schools known as yeshivas".

[25] In June 2018, Democratic State Committee members approved an informal resolution recommending Felder's ouster from the party.

[25] Former state Democratic Chairman Jay Jacobs, speaking to reporters afterward, reiterated their exasperation with Felder:[25] It's my belief is that the party is not merely a vessel for someone's personal ambition to take someone from where they are to where they want to be.

[42] On January 9, 2019, the Daily News reported that Felder had decided against caucusing with the Republicans, and was looking forward to rejoining the Senate Democrats "'at the appropriate time'".